modify package:mtl

Monadic state transformer. Maps an old state to a new state inside a state monad. The old state is thrown away.

Main> :t modify ((+1) :: Int -> Int)
modify (...) :: (MonadState Int a) => a ()

This says that modify (+1) acts over any Monad that is a member of the MonadState class, with an Int state.

A different MonadError analogue to the withExceptT function. Modify the value (and possibly the type) of an error in an ExceptT-transformed monad, while stripping the ExceptT layer. This is useful for adapting the MonadError constraint of a computation. For example:

data DatabaseError = ...

performDatabaseQuery :: (MonadError DatabaseError m, ...) => m PersistedValue

data AppError
= MkDatabaseError DatabaseError
| ...

app :: (MonadError AppError m, ...) => m ()

Given these types, performDatabaseQuery cannot be used directly inside app, because the error types don't match. Using modifyError, an equivalent function with a different error type can be constructed:

performDatabaseQuery' :: (MonadError AppError m, ...) => m PersistedValue
performDatabaseQuery' = modifyError MkDatabaseError performDatabaseQuery

Since the error types do match, performDatabaseQuery' _can_ be used in app, assuming all other constraints carry over. This works by instantiating the m in the type of performDatabaseQuery to ExceptT DatabaseError m', which satisfies the MonadError DatabaseError constraint. Immediately, the ExceptT DatabaseError layer is unwrapped, producing Either a DatabaseError or a PersistedValue. If it's the former, the error is wrapped in MkDatabaseError and re-thrown in the inner monad, otherwise the result value is returned.

A variant of modify in which the computation is strict in the new state.